(recursive acronym: "The ACME Crossassembler for Multiple Environments")
A 6502 crossassembler from Smørbrød Software (Marco Baye), distributed under GPL. It is portable and runs on various platforms (*NIX, MS-DOS, AmigaOS, OS/2 and RISC OS, the original platform where it was written on). It supports many kinds of 6502 variants (6502, 65c02, 65816 - though it will not produce files bigger than 64k!), and supports for illegal opcodes are being worked on.
It appears to be very well-featured: it allows complicated preprocessing (conditional and looping code, inclusion of other source files and binaries, math parsing), supports global and local labels, and has powerful macro support. It also appears to support position-independent code (but will not make linkable objects - hmmmm). It also seems to have ASCII-to-PETSCII/screencodes conversion built in. And, according to the hype, it is also fast.